1. Field of the Invention
Computer vision applications involving digital optical metrology provide for precisely imaging test pieces for purposes of measurement or comparison. Calibration techniques accommodate imaging system errors.
2. Description of Related Art
Nonlinear field of view distortions are particularly troublesome for computer vision applications that require precise measurements or comparisons of imaged test objects. The distortions do not affect image quality, such as the sharpness of the image, but do affect image geometry, which can complicate the accurate determination of size, shape, and location of the imaged test objects.
For example, curvilinear distortions are manifest as so-called “barrel” or “pincushion” distortions. Focal length varies as a function of radial distance from a center of the field. Straight tangential lines tend to curve, while radial lines remain straight. “Barrel” distortion decreases focal length as the field angle increases, and “pin cushion” distortion increases focal length as the field angle increases. Other distortions in which the resulting image is not a true-to-scale reproduction of an object include radial distortion in which magnification varies from the center of the field, anamorphic distortions in which magnification varies with direction, and tangential distortions such as resulting from centration errors.
Among the most significant problems caused by such nonlinear field of view distortions for the imaging of test objects are size, shape, and location distortions. The same test object can appear to be differently sized or shaped depending upon its position within the field of view. The apparent distance an object moves within the field of view can differ from the actual distance that the object moves. For example, the location of an object's edge from the center of the field of view can differ from the actual location of the object's edge depending upon such factors as the position and orientation of the object's edge within the field of view.
Solutions for measuring and correcting for such nonlinear field of view distortions include the use of calibrated artifacts having known size and shape. The calibrated artifacts are moved to different locations within the field of view and deviations in the measured size and shape are used to generate local error corrections within the field of view. For digital cameras, lookup tables are created for recording pixel-by-pixel corrections.
Another technique for compensating for nonlinear field of view distortions involves placing a calibrated grid within the field of view. Typically, the calibrated grid fills the entire field of view, and the imaged locations of the grid lines are compared to their known locations to generate corrections required for constructing a more accurate image of the calibrated grid.
The manufacture and certification of calibrated artifacts and grids involves considerable expense. The accuracy with which the calibrated artifacts are known affects the accuracy of the corrections that can be made to compensate for the distortions of the field of view. Multiple artifacts and grids are sometimes needed for measuring distortions of the field of view at different magnifications.